1. Description of the Related Art
After Light Emitting Diode (LED) with the features of low power consumption and high performance hits the lighting market, related manufacturers spare no effort on the subject of controlling the illumination brightness, work efficiency, and service life of LED lamps. Most conventional LED lamps adopt a control circuit with a constant current architecture and connect an LED to an N-Type Metal Oxide Semiconductor Field Effect Transistor (N-MOSFET) and a current resistor in series with, and the current resistor limits the output current passing through the LED. The current resistor receives the output current and forms a voltage drop value to be fed back to an operational amplifier at both terminals of the current resistor. After the operational amplifier uses a reference voltage value to compare the voltage drop value, and a negative feedback circuit formed by connecting the operational amplifier, the N-MOSFET and the current resistor, such that the voltage drop values at both terminals of the current resistor are constant and equal to the referenlce voltage value to keep the output current constant, so as to limit the Total Harmonic Distortion (THD) within a range to facilitate the product to be sold in markets at different places. However, the total output current in a 180-degree phase sine wave period of an AC voltage of an external power supply is non-constant and varies with an increase of the voltage value of the AC voltage, and thus the component specification requirement of the LED becomes higher.
In addition, the mains power of different countries in the world are different, or even the mains power are the same, persons having ordinary skill in the art should know that the voltage value of the AC voltage is not stable and varies with time period. As a result, the output current or voltage will vary and affect the efficiency and THD value of the whole circuit. The greater the total output power, the less controllable the THD value. As a result, the THD value cannot be maintained stably within the specification, and the product quality will be unstable and unable to launch into some markets. The economic value of the industry will be reduced.
With reference to FIG. 1 for a conventional multi-sectional driving circuit 1, the multi-sectional driving circuit 1 comprises a detection part 10, a comparison part 11, a regulation part 12 and a current resistor 13, and the serially connected LEDs are divided into a plurality of small strings, and the detection part 10 is provided for detecting the voltages at the nodes between the strings to produce a detected value, and the current resistor 13 receives the current passing through the LED and forms an operating voltage value at both terminals of the current resistor 13. The comparison part 11 comprises a positive input terminal for receiving a reference value (Vref). When the detected value is received, the reference value is compared with the operating voltage value to drive the regulation part 12 to control the flowing range of the output current and then drive the strings to emit light to form an output current waveform as shown in FIG. 2. In a 180-degree phase sine wave period of the AC voltage, the output current is electrically conducted in a plurality of cycle sections, and the current value of each cycle section varies with the sinusoidal change of the AC voltage, so as to improve the power utilization. However, such method incurs complicated circuit architecture and high cost, and it still cannot overcome the problem that the total current of the output current varies with the sinusoidal change of the AC voltage. As a result, the lamps required LED components of high specification.
In view of the aforementioned problems, it is an important subject of the present invention to improve the operation of the driving circuit and output a constant total amount of current by affecting the conduction cycle of the output current to output a constant total amount of current, so that the whole circuit conversion efficiency is constant and the utilization of components is improved.
2. Summary of the Invention
Therefore, it is a primary objective of the present invention to overcome the drawbacks of the prior art by providing a high performance linear LED driving circuit, such that when the mains power varies, the duty cycle of a two-stage conducted output current is modulated to achieve the effects of a stable total output current and a high efficiency.
To achieve the aforementioned objective, the present invention discloses a high performance linear LED driving circuit for converting an AC voltage of an external power supply into an output current of a direct current (DC) and outputting the DC current to at least one LED and having a detection unit, a control unit and a current unit, and the detection unit being electrically coupled to the control unit and the LED, and the current unit being electrically coupled to the control unit and the LED, characterized in that the total current of the output current in a 180-degree phase sine wave period of the AC voltage is comprised of a first working section, an energy saving section and a second working section, and the detection unit is provided for detecting at least one selected from a voltage value of the AC voltage and a current value of the output current to form a detection signal, so as to drive the control unit to modulate the duty cycle and the current value of the first working section, the energy saving section and the second working section, and the amount of current of the first working section and the amount of current of the second working section are greater than the amount of current of the energy saving section.
Wherein, the control unit modulates the cycle time and current value of the first working section and the second working section according to the detection signal, so that the ratio of the total input power Pin of the external power supply to the total output power Pout of the high performance linear LED driving circuit is a constant C, and Eff=Pout/Pin=C, so as to achieve the effect of a constant efficiency. The total current of the energy saving section is less than 20% of the sum of the total current of the first working section and the total current of the second working section.
In addition, the control unit comprises a controller, a reference voltage regulator, a control comparator and a switch, and the controller is electrically coupled to the detection unit, the reference voltage regulator and the switch, and the control comparator is electrically coupled to the reference voltage regulator, the switch and the current unit, and the switch is electrically coupled to the current unit. In an implementation mode, the detection unit comprises a detection resistor and a detection comparator, and an output terminal of the detection comparator is electrically coupled to the controller, and the detection resistor is electrically coupled to an input terminal of the detection comparator and a negative electrode of the LED, and a voltage value of the negative electrode of the LED is detected to produce a detected voltage value, and the detection comparator uses a reference voltage to compare with the detected value to form the detection signal provided for the controller to change a status of the switch according to the voltage value of the detection signal and affect the control comparator to module the duty cycle of the current unit and the magnitude of the output current passing through the LED, so as to achieve the effect of a constant current.
In another implementation mode, the detection unit comprises a detection resistor and a detection comparator, and the detection resistor is connected to a negative electrode of the LED and the current unit is series, and an input terminal of the detection comparator is coupled both terminals of the detection resistor and an output terminal of the detection comparator is electrically coupled to the controller, and the detection resistor receives the output current passing through the LED to form a voltage drop which is provided for the detection comparator to drive the controller to change a status of the switch and affect the control comparator to modulate the duty cycle of the current unit and the magnitude of the output current of the LED, so as to achieve the effect of a constant current.
The detection unit comprises a detection resistor and a detection comparator, and an output terminal of the detection comparator is electrically coupled to the controller, and the detection resistor is electrically coupled to an input terminal of the detection comparator and a positive electrode of the LED, and the AC voltage is detected and rectified to form a detected voltage value, and the detection comparator uses a reference voltage to compare and form the detection signal which is provided for the controller to change a status of the switch according to a conduction time of the detection signal and affect the control comparator to modulate the duty cycle of the current unit and the magnitude of an output current passing through the LED, so as to achieve the effect of a constant current.
In addition, the detection unit comprises a Zener diode and at least one current reduction resistor, and the control unit comprises a control comparator, and an output terminal of the control comparator is coupled to the current unit, and a negative electrode of the Zener diode is electrically coupled to a positive electrode of the LED, and the positive electrode is electrically coupled to the current reduction resistor and the control comparator, and the current reduction resistor detects the rectified AC voltage to form the detection signal which is provided for the control comparator to use a reference voltage for comparison and then change the duty cycle of the current unit.
The LED comprises a capacitor of a large capacitance value connected in parallel to the LED to improve the current constancy of the output current to prevent a strobe of the LED.
In summation, the present invention breaks through the traditional thought and outputs a current with double wave sections in a 180-degree phase sine wave period of the AC voltage, and the duty cycle and current value of the first working section and the second working section vary with a change of the AC voltage, so that the total amount of current of the output current is maintained constant to achieve the effect of a constant output power, so as to improve the power conversion efficiency of the whole circuit and meets the industrial requirements. Therefore, the present invention may be used for 110V LED lamps with 80V or 90V LEDs without requiring the use of the 130V LEDs, and thus greatly lowering the requirement of the driving voltage of the LED, improving the utilization of the LED, and reducing the cost of the lamp.